1. Field of the Invention
The present invention relates generally to the formation of solder joints to electrodes on a substrate, such as solder bumps used in conjunction with solder bump array integrated circuit packages (i.e., flip chip assemblies, chip scale packages, and ball grid array structures), and more particularly, to a method of reinforcing such solder bumps in a manner that reduces failures due to temperature cycling.
2. Description of the Related Art
Surface mount technology using solder bump array integrated circuit packages (hereinafter referred to as an IC package) is well known in the semiconductor industry for simplifying the packaging and interconnection of integrated circuits. Typically, a series of circular (as viewed from above, or semi-spherical in three dimensions) solder bumps are formed upon the surface of an IC package or other substrate in electrical contact with active or passive devices formed or attached upon such substrate. Such solder bumps are then aligned with pads formed in a corresponding pattern upon a second substrate to which the first substrate is to be mounted. The aforementioned integrated circuit packages are typically produced with die that are scribed from a semiconductor wafer. During processing, such semiconductor wafer has an upper active surface through which impurities are introduced, by chemical diffusion and/or implantation, to form individual transistors and other electronic components. Metallization layers are also patterned upon the upper, or active, surface of such semiconductor wafer to electrically interconnect the electrodes of the various devices formed in such semiconductor wafer. For flip chips, the upper active surface of scribed integrated circuit die are inverted, or flipped, in order to be solder connected to an underlying patterned substrate. Heating of the solder bumps to their xe2x80x9creflowxe2x80x9d temperature melts the solder, and the xe2x80x9creflowxe2x80x9d of the solder joins the flip chip electrically and mechanically with the underlying patterned support substrate. The use of solder bumps to interconnect such flip chip integrated circuits to underlying support substrates is disclosed, for example, within U.S. Pat. No. 5,261,593 to Casson, et al.; within U.S. Pat. No. 5,220,200 to Blanton; within U.S. Pat. No. 5,547,740 to Higdon, et al.; and within U.S. Pat. No. 5,564,617 to Degani, et al.
Often, the IC package and the supporting substrate (i.e., printed circuit board, ceramic substrate, etc.) to which it is joined have different CTEs (Coefficients of Thermal Expansion). During thermal cycling, the CTE mismatch will create thermal strains/stresses which will fracture the solder bumps used to join the IC package to the supporting substrate, causing the circuitry to fail. Two methods have been used to improve the reliability of solder joints that use solder balls. A first common technique for IC package joints is to utilize an underflll encapsulant that flows between the IC package and the supporting substrate, filling the space around the solder bumps, as well as physically connecting the IC package surface to the substrate surface, to constrain thermal expansion differences between the IC package and the substrate, thereby improving the solder joint fatigue life. While the use of such underfill material improves the fatigue life of the solder joints, the application of this underfill is often perceived as an expensive process that is not consistent with standard surface mount technology manufacturing processes.
The second technique that has been mentioned in the technical literature has been to partially encapsulate the base of the solder balls to minimize the stresses to which the base of the solder ball is subjected. The base of the solder ball is typically where a solder fatigue crack will initiate and propagate. One method of stabilizing the base of the solder ball has been described by ceramics supplier Kyocera in conjunction with a DBGA (Dimpled Ball Grid Array) process wherein an extra layer of ceramic is placed upon a multi-layer ceramic substrate prior to solder ball attachment; the extra layer of ceramic has holes formed therein over the solderable pads. A second method of stabilizing the base of the solder balls has been described by NEC Corporation using a resin reinforcement layer that is dispensed around the solder balls following solder ball attachment to the solderable pads. This NEC technique is described in a technical paper entitled xe2x80x9cNew Technology for enhancing Solder Reliability of D2BGA (Die Dimension BGAxe2x80x9d, by Kazutaka Shoji, et al., presented in 1997. Related subject matter is disclosed within U.S. Pat. No. 5,847,456, issued on Dec. 8, 1998, to Shoji, and within U.S. Pat. No. 5,869,904, issued on Feb. 9, 1999, to Shoji; both of these patents are assigned to NEC Corporation of Tokyo, Japan.
However, the methods that have been used in the past to stabilize solder ball joints, and thereby improve the reliability of such solder ball joints, are generally complex and significantly increase manufacturing expense. Typical techniques of applying and patterning a solder mask prior to ball placement, while feasible, add cost and potentially impact package reliability and susceptibility to moisture damage. Moreover, the application of underfill layers or reinforcement resins following the ball attachment process do not serve to help maintain the shape of the solder balls during the initial ball attachment process. In addition, the above-described techniques for applying underfill layers or resin reinforcements following the ball attachment process also add significantly to manufacturing expense and process complexity.
Accordingly, it is an object of the present invention to provide an improved method for forming solder bumps for flip chip integrated circuits, chip scale packages, and ball grid array structures, which is consistent with standard surface mount technology manufacturing processes, and which provides additional mechanical support to the base of solder balls to protects the solder balls from fatigue induced by thermal coefficient differentials, thereby increasing the reliability of such solder balls, without significantly increasing manufacturing costs or processing complexity.
It is another object of the present invention to provide such a method which can be carried out at the wafer processing level, or in the case of ball grid arrays or chip scale packages, in matrix/multi-up configurations, or on individual packages.
Still another object of the present invention is to provide such a method which combines the step of reinforcing the base of a solder ball with the step of applying a flux to solder ball pads prior to ball attachment.
Yet another object of the present invention is to provide such a method which can be applied to a variety of electronic packaging applications, including without limitation, Ball Grid Array (BGA), Chip Scale Package (CSP) and flip chip structures.
A further object of the present invention is to provide such a method which helps to maintain the original shape of solder balls during ball attachment and during subsequent reflow operations.
These and other objects of the present invention will become more apparent to those skilled in the art as the description of the present invention proceeds.
Briefly described, and in accordance with a preferred embodiment thereof, the present invention relates to a method of forming a solder-bumped structure upon a substrate, such as an IC package, by initially providing a first conductive solder bump pad on the surface of the substrate. An uncured polymer material is then applied over the solder bump pad. This uncured polymer material is preferably a no-flow underfill material of a type which achieves a fluxing action by reducing metal oxides that may have formed upon the solder bump pad or upon the base region of a solder ball that is to be joined with such solder bump pad. The base region of a first pre-formed solder ball is then advanced into the uncured polymer material and onto the first solder bump pad. The resulting assembly is then heated to the characteristic reflow temperature of the first solder ball to join the base region of the first solder ball to the first conductive solder bump pad in the form of a metallurgical joint. This heating cycle also serves to at least partially cure the polymer material surrounding the base region of the first solder ball. The polymer material thereby forms a support ring, or collar, around the base of the solder ball to stabilize such base region, and to guard against solder fatigue failure.
In the preferred embodiment of the present invention, the substrate includes two or more of such conductive solder bump pads formed upon the surface of the substrate, and the uncured polymer material is applied over each of such solder bump pads. Pre-formed solder balls are placed over each of such solder bump pads, the base region of each such solder ball advancing into the uncured polymer material and onto its respective solder bump pad. The base region of each such solder ball is then joined to its respective solder bump pad during the reflow heating cycle, during which the polymer material surrounding the base region of each solder ball is at least partially cured.
While the uncured polymer material can be applied as a continuous layer across the substrate, it is preferred that the uncured polymer material be applied as patterned regions each overlying one of the solder bump pads. To achieve such a patterned application of the uncured polymer material, the uncured polymer material can be printed onto the substrate through a patterned screen or stencil. The uncured polymer material preferably has a viscosity of at least 30,000 centipoise for optimal printing, and to avoid significant spreading away from the solder bump pad following application.
As alternatives to application of the polymer material by printing methods, the polymer material can also be applied to the solder bump pads by dispense techniques, including jetting technologies, or by the pin transfer method. A further embodiment of the present invention involves grasping each solder ball prior to placement, applying the polymer material to the base region of the grasped solder ball, and then placing the grasped solder ball onto its associated solder bump pad, thereby simultaneously applying the polymer material to the solder bump pad and placing the pre-formed solder ball onto the solder bump pad.
Should the heating cycle used to join the solder balls to their respective solder bump pads at the reflow temperature prove to be insufficient to complete the curing of the polymer material, a further heating cycle is added to finish curing the polymer material.
The method of the present invention improves solder joint life by enhancing the mechanical support of the base of the solder joint, thereby reducing fatigue failures induced by thermal stresses. The aforementioned method of producing such a polymer collar can be achieved at a much lower manufacturing cost as compared with known techniques for encapsulating solder joints. The polymer material that forms the collar doubles as a fluxing agent, and further serves to help maintain the shape of the solder ball during ball attachment operations.